1. Field of the Invention
This invention relates to an improvement in or relating to differential-type protective relays suitable for application to electric power systems so as to protect the buses of the electric power systems or various apparatus, equipment, devices and/or the like connected to the buses.
2. Description of the Prior Art
Taking by way of example protective relays adapted to protect buses, there have conventionally been known such protective relays as illustrated diagrammatically in FIG. 1.
In the figure, there are shown a bus 1, a main relay 2, outgoing lines 11, 21, current transformers 12, 22 provided respectively with the outgoing lines 11, 21, input devices or transformer units 13, 23 connected respectively to the current transformers 12, 22, differential input transformers 13-1, 23-1, restraining input transformers 13-2, 23-2, restraining voltage rectifiers 13-3, 23-3, an operating voltage input transformer 2-1, a restraining/controlling input transformer 2-2, an operating voltage rectifier 2-3, a restraining/controlling rectifier 2-4, an operating voltage output resistor 2-5, a restraining/controlling voltage output resistor 2-6, a restraining voltage output resistor 2-7, a restraining voltage stretching capacitor 2-8 and a level detector 2-9.
The operation of the above protective relay will next be described. In the circuit illustrated in FIG. 1, differential currents I.sub.D which have been induced respectively in the secondary windings of the differential input transformers 13-1, 23-1 are fed to the main relay 2. The differential current I.sub.D fed to the main relay 2 is then caused to flow through the operating voltage input transformer 2-1 and operating voltage rectifier 2-3 to the operating output resistor 2-5, where it produces an operating voltage .vertline.E.sub.O .vertline.. At the same time, the differential current I.sub.D fed to the main relay 2 is also caused to pass via the restraining/controlling input transformer 2-2 and restraining/controlling rectifier 2-4 to the restraining/controlling voltage output resistor 2-6, where it produces a restraining/controlling voltage .vertline.E.sub.P .vertline.. On the other hand, by way of the restraining input transformers 13-2, 23-2 and restraining voltage rectifiers 13-3, 23-3 of the input transformer units 13, 23, the maximum currents of the secondary currents of the current transformers 12, 22 connected to their respective terminals are introduced as a terminal restraining voltage .vertline.E.sub.T .vertline. to the main relay 2. The terminal restraining voltage E.sub.T and the above-mentioned restraining/controlling voltage .vertline.E.sub.P .vertline. are instantaneously compared in value, thereby outputting the remainder obtained by subtracting the restraining/controlling voltage .vertline.E.sub.P .vertline. from the terminal controlling voltage .vertline.E.sub.T .vertline. as a final restraining voltage .vertline.E.sub.R .vertline. to the restraining voltage output resistor 2-7. The restraining voltage .vertline.E.sub.R .vertline. is stretched over a suitable period of time by the restraining voltage stretching capacitor 2-8. The main relay 2 is designed in such a way that it permits actuation and output of the level detector 2-9 when the operating voltage .vertline.E.sub.O .vertline. is high relative to the restraining voltage .vertline.E.sub.R .vertline.. When an internal fault has been developed in the bus, the differential current I.sub.D is generated and the operating voltage .vertline.E.sub.O .vertline. is then produced at the operating voltage output resistor 2-5 of the main relay 2. On the other hand, the terminal restraining voltage .vertline.E.sub.T .vertline. is supplied to the main relay 2. However, the restraining/controlling voltage .vertline.E.sub.P .vertline. occurred from the differential current I.sub.D has already been impressed to the both ends of the restraining/controlling voltage output resistor 2-6 at this stage. Thus, the restraining/controlling voltage .vertline.E.sub.P .vertline. acts in such a way that it suppresses the terminal restraining voltage .vertline.E.sub.T .vertline.. In case of an internal fault, the secondary currents of the current transformers 12, 22 provided at the side of the incoming terminals have waveforms similar to the differential current I.sub.D. Where many incoming terminals are provided, the differential current I.sub.D is higher than any terminal current. Hence the restraining/controlling voltage .vertline.E.sub.P .vertline. occurred in proportion to the differential current I.sub.D becomes higher than the terminal restraining voltage .vertline.E.sub.P .vertline. generated in proportion to the maximum value of the secondary currents of the current transformers 12, 22 provided at the side of the incoming terminals. Correspondingly, the terminal restraining voltage .vertline.E.sub.T .vertline. is always eliminated the restraining voltage .vertline.E.sub.P .vertline. is lowered to zero, thereby ensuring fail-free actuation of the main relay 2.
Turning next to an external fault, the current transformers 12, 22 provided at the side of the incoming terminals are saturated to generate an erroneous differential current I.sub.D, whereby producing in some instances the operational voltage .vertline.E.sub.O .vertline. at the operating voltage output resistor 2-5 of the main relay 2. Therefore, the restraining voltage .vertline.E.sub.R .vertline. is required to avoid any malfunction. As is well known in the art, a current transformer is extremely susceptible to saturation when a current containing a dc component is fed thereto. Therefore, such a current transformer is most likely to undergo erroneous actuation where a current contains an attenuating transient direct current component at the time of an external fault. The response of a conventional relay under the above-described situation will next be described with reference to a waveform diagram illustrated in FIG. 2.
In FIG. 2, the waveform A corresponds to a summed current from the current transformers provided at the side of the incoming terminals. The dotted line indicates a current under unsaturation, while the solid wave represents an actual secondary current of each of the current transformers. Since more than one incoming terminal is provided, a fault current is divided to the current transformers. The current transformers are thus saturated respectively by direct currents, although their degrees of saturation are low. The waveform B indicates the waveform of a secondary current induced by each of the current transformers provided at the side of the incoming terminals. The dotted line corresponds to unsaturation, while the solid line indicates an actual secondary current of each of the current transformer. Since the outgoing terminal is concentrated with the fault current, the alternative-current saturation becomes dominant. The waveform C corresponds to the terminal restraining voltage .vertline.E.sub.T .vertline., which is proportional to the maximum value among all the secondary currents. Supposing that two or more terminals are provided at the incoming side and no restraining force is expected from the current transformers provided at the side of the incoming terminals, the terminal restraining voltage derived only from the secondary currents of the current transformers provided at the side of the incoming terminals is shown there. Therefore, the waveform C has been produced from the waveform B. The voltage output from the restraining input transformer of the input transformer unit, which restraining input transformer generates the terminal restraining voltage .vertline.E.sub.T .vertline., has a waveform obtained by differentiating the primary current and the dc component of the primary current has been eliminated, because the restraining input transformer is composed of a gapped transformer in order to avoid direct-current saturation. The waveform D represents the erroneous differential current I.sub.D and has been obtained by subtracting the waveform B from the waveform A. The negative erroneous differential current of the waveform D is the direct-current saturation error of the transformers, which are provided at the side of the outgoing terminals, in the unsaturated range of the same transformers. The negative error of the waveform D becomes greater as the degrees of direct-current saturation of the transformers provided the side of the incoming terminals increase. The waveform E corresponds to the restraining/controlling voltage .vertline.E.sub.P .vertline.. It has been produced from the waveform D. It is necessary to preset the phase of the voltage .vertline.E.sub.T .vertline. equal to the phase of the voltage .vertline. E.sub.P .vertline. because an operation has to be performed to eliminate the terminal restraining voltage .vertline.E.sub.T .vertline. in the event of an internal fault as described above. Accordingly, the restraining/controlling input transformer 2-2 is also composed of a gapped transformer. The secondary output voltage of the transformer 2-2 has a waveform obtained by differentiating the current input to the transformer 2-2. The waveform F represents the restraining voltage .vertline.E.sub.R .vertline.. This restraining voltage .vertline.E.sub.R .vertline., which is hatched, has been obtained by stretching by the capacitor 2-8 a voltage obtained by subtracting the waveform E from the waveform C and occurred at both ends of the restraining output resistor 2-7. The waveform G corresponds to the operational voltage .vertline.E.sub.O .vertline. and is proportional to a waveform obtained by subjecting the waveform D to full-wave rectification.
As described above, the erroneous differential current I.sub.D and terminal restraining voltage .vertline.E.sub.T .vertline. which are impressed to the main relay 2 when the saturation of the current transformers has occurred due to an external fault have such waveforms as indicated respectively by the waveform C and waveform D of FIG. 2. As a result, the operational voltage .vertline.E.sub.O .vertline. and restraining voltage .vertline.E.sub.R .vertline. of the main relay 2 have waveforms similar to the waveform F and waveform G. The voltage applied to the level detector 2-9, which voltage governs the operation of the main relay 2, is thus the voltage difference between the waveform G and waveform F. Therefore, the restraining voltage indicated by the hatches on the waveform F serves as an actual restraining force in order to avoid any malfunction. The magnitude of the restraining voltage and the stretching characteristic of the capacitor 7 determine the performance for the prevention of malfunctions.
Since conventional protective relays are constructed as described above, the level of the restraining voltage .vertline.E.sub.R .vertline. upon occurrence of an external fault is affected greatly by the characteristic of the capacitor 2-8. When a troubled current has long dc attenuating time and the second or third wave component of the secondary current of each of the current transformer provided at the side of the incoming terminals is small, the second or third wave component of the terminal restraining voltage .vertline.E.sub.T .vertline. becomes smaller. As a result, the restraining voltage .vertline.E.sub.R .vertline. becomes smaller, thereby developing the danger of a malfunction. Furthermore, in order to prevent malfunctions upon occurrence of an external fault, it is necessary to use a gapped transformer as the terminal restraining input transformer of the input transformer unit so that the input transformer is not saturated with a direct current. On the other hand, it is also required to compose the restraining/controlling input transformer with a gapped transformer in order to eliminate the terminal restraining voltage .vertline.E.sub.T .vertline. without failure upon occurrence of an internal fault, whereby to ensure the perfect matching in phase. However, this has resulted in another drawback because it is difficult to match the time constant (L/R) of a circuit with that of another circuit by adjusting the time constants of parts making up both circuits.